Numerous proposals are known from the prior art to provide olefin polymerization catalysts by combining a solid component comprising at least magnesium, titanium and chlorine with an activating organoaluminum compound. These may be referred to as supported coordination catalysts or catalyst systems. The activity and stereospecific performance of such compositions is generally improved by incorporating an electron donor (Lewis base) in the solid component and by employing as a third catalyst component an electron donor which may be complexed in whole or in part with the activating organoaluminum compound.
For convenience of reference, the solid titanium-containing constituent of such catalysts is referred to herein as "procatalyst", the organoaluminum compound, whether used separately or partially or totally complexed with an electron donor, as "cocatalyst", and the electron donor compound, whether used separately or partially or totally complexed with the organoaluminum compound, as "selectivity control agent" (SCA).
Supported coordination catalyst of this type are disclosed in numerous patents. The catalyst systems of this type which have been disclosed in the prior art generally are able to produce olefin polymers in high yield and, in the case of catalysts for polymerization of propylene or higher alpha-olefins, with high selectivity to stereoregular polymer. However, further improvements in productivity at high stereoregularity are still being sought.
The objective of workers in this art is to provide catalyst systems which exhibit sufficiently high activity to permit the production of polyolefins in such high yield as to obviate the necessity of extracting residual catalyst components in a deashing step. In the case of propylene and higher olefins, an equally important objective is to provide catalyst systems of sufficiently high selectivity toward isotactic or otherwise stereoregular products to obviate the necessity of extracting atactic polymer components.
Although many chemical combinations provide active catalyst systems, practical considerations have led the workers in the art to concentrate on certain preferred components. The procatalysts typically comprise magnesium chloride, titanium chloride, generally in tetravalent form, and as electron donor an aromatic ester such as ethyl benzoate or ethyl-p-toluate. The cocatalyst typically is an aluminum trialkyl such as aluminum triethyl or aluminum tri-isobutyl, often used at least partially complexed with selectivity control agent. The selectivity control agent typically is an aromatic ester such as ethyl-paramethoxybenzoate(ethyl anisate) or methyl-p-toluate.
While the selection of cocatalyst and selectivity control agent affects the performance of those catalyst systems, the component which appears to be subject to most significant improvement with respect to activity and productivity of the system is the procatalyst.
Preferred methods of preparing such procatalysts are claimed in U.S. Pat. Nos. 4,329,253; 4,393,182; 4,400,302; 4,328,328; 4,478,952 and 4,414,132. These procatalysts are highly active and stereospecific. The typical manner of preparing such procatalysts involves the reaction of the magnesium compound, titanium tetrachloride and electron donor in the presence of a halohydrocarbon. The resulting solid particles are then contacted with additional quantities of TiCl.sub.4 and are completed by washing off excess TiCl.sub.4 using light hydrocarbons (e.g., isooctane and isopentane) and drying.
The procatalysts described above have excellent polymerization activity (polymer yield) and stereospecific performance (isotactic content). However, for some applications the polymer morphology is not ideal. In olefin polymerization, polymer morphology is known to be a replica of catalyst morphology. Still further, the procatalyst morphology also depends upon the morphology of the starting magnesium compound. Accordingly, if one desires to have optimal catalyst morphology (e.g. spheroidal particles), then it is desirable to employ starting magnesium compounds of the same morphology.
A number of different approaches to improved morphology are suggested in the patent literature. One approach, disclosed in GB Pat. No. 2,101,610, involves reacting a solid particulate material with an organic magnesium compound, treating the supported magnesium composition with oxygen, carbon dioxide or a hydroxyl compound, reacting the treated product with a carbonyl compound and simultaneously or subsequently reacting with a transition metal compound. Another approach, disclosed in U.S. Pat. No. 4,465,783, involves the spray drying of a transition metal composition, or a support for a transition metal compound, suspended in a liquid medium. Still another method is disclosed in DE Pat. No. 2,839,188, where solid magnesium dialkoxide particles are dispersed into a suitable liquid phase, followed by spray-drying. However, the process of the '188 patent is not attractive as the dispersed solid particles will tend to clog the fine orifices of the spray-drying equipment and will foul the pumping and metering systems. In U.S. Pat. No. 4,540,679, use is made of a magnesium hydrocarbyl carbonate support. In the '679 patent, a suspension of magnesium alcoholate with carbon dioxide is reacted with a transition metal component to precipitate a "magnesium hydrocarbyl carbonate" support. The patentees use a number of techniques, including prepolymerization and the use of triethyl aluminum (TEA) to remove ethanol, to improve productivity. However, these techniques are not desirable--e.g., prepolymerization is an additional step and the addition of TEA adds ash to the polymer product.
The above-mentioned approaches to morphology control all depend upon starting from roughly spherical amorphous, non-stoichiometric shapes. A new method to improve morphology is greatly desired.
A new approach has now been found, unique in that the magnesium precursor is a crystalline compound of well defined stoichiometry and shape, and that permits the preparation of procatalyst particles having not only excellent productivity and selectivity, but also possessing excellent morphology.